comment regarding double timestamps; and, infinite timestamps and NaN

selfuncs.c convert_to_scalar() says:

|* The several datatypes representing absolute times are all converted
|* to Timestamp, which is actually a double, and then we just use that
|* double value.  Note this will give correct results even for the "special"
|* values of Timestamp, since those are chosen to compare correctly;
|* see timestamp_cmp.

But:
https://www.postgresql.org/docs/10/release-10.html
|Remove support for floating-point timestamps and intervals (Tom Lane)
|This removes configure's --disable-integer-datetimes option. Floating-point timestamps have few advantages and have
notbeen the default since PostgreSQL 8.3.
 
|b6aa17e De-support floating-point timestamps.
|configure                                                    | 18 ++++++------------
|configure.in                                                 | 12 ++++++------
|doc/src/sgml/config.sgml                                     |  8 +++-----
|doc/src/sgml/datatype.sgml                                   | 55
+++++++++++--------------------------------------------
|doc/src/sgml/installation.sgml                               | 22 ----------------------
|src/include/c.h                                              |  7 ++++---
|src/include/pg_config.h.in                                   |  4 ----
|src/include/pg_config.h.win32                                |  4 ----
|src/interfaces/ecpg/include/ecpg_config.h.in                 |  4 ----
|src/interfaces/ecpg/include/pgtypes_interval.h               |  2 --
|src/interfaces/ecpg/test/expected/pgtypeslib-dt_test2.c      |  6 ++----
|src/interfaces/ecpg/test/expected/pgtypeslib-dt_test2.stdout |  2 ++
|src/interfaces/ecpg/test/pgtypeslib/dt_test2.pgc             |  6 ++----
|src/tools/msvc/Solution.pm                                   |  9 ---------
|src/tools/msvc/config_default.pl                             |  1 -
|15 files changed, 36 insertions(+), 124 deletions(-)

It's true that convert_to_scalar sees doubles:
|static double
|convert_timevalue_to_scalar(Datum value, Oid typid, bool *failure)
|{
|        switch (typid)
|        {
|                case TIMESTAMPOID:
|                        return DatumGetTimestamp(value);

But:
$ git grep DatumGetTimestamp src/include/
src/include/utils/timestamp.h:#define DatumGetTimestamp(X)  ((Timestamp) DatumGetInt64(X))

So I propose it should say something like:

|* The several datatypes representing absolute times are all converted
|* to Timestamp, which is actually an int64, and then we just promote that
|* to double.  Note this will give correct results even for the "special"
|* values of Timestamp, since those are chosen to compare correctly;
|* see timestamp_cmp.

That seems to be only used for ineq_histogram_selectivity() interpolation of
histogram bins.  It looks to me that at least isn't working for "special
values", and needs to use something other than isnan().  I added debugging code
and tested the attached like:

DROP TABLE t; CREATE TABLE t(t) AS SELECT generate_series(now(), now()+'1 day', '5 minutes');
INSERT INTO t VALUES('-infinity');
ALTER TABLE t ALTER t SET STATISTICS 1;
ANALYZE t;
explain SELECT * FROM t WHERE t>='2010-12-29';